As an emission type electronic displaying device, there is an electroluminescent display (hereinafter referred to as ELD). As elements constituting the ELD, there are mentioned an inorganic electroluminescent element and an organic electroluminescent element (hereinafter referred to as organic EL element). The inorganic EL element has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the element. An organic EL element has a structure in which a light emission layer containing a light emission compound is provided between a cathode and an anode, and an electron and a hole are injected into the light emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the element can emit light by applying a relatively low voltage of from several volts to several decade volts. The element has a wide viewing angle and a high visualization property, since the element is of self light emission type. Further, the element is a thin, complete solid device, and therefore, the element is marked from the viewpoint of space saving and portability. Further, the organic EL element is expected to be applied to a backlight of a liquid crystal display and an illuminating device as well as a display of self light emission type.
However, in the organic EL element to be put into practical use, development of an organic EL element efficiently emitting light with high luminance at low power consumption is desired.
The light emission efficiency of the organic EL element is divided into an inner efficiency and an external efficiency (or light extraction efficiency). In the organic EL element, the surface of each constituent layer such as a substrate, an electrode, or a light emission layer is optically smooth, and the refractive index of the substrate is around 1.5 and the refractive index of materials for the light emission layer is generally high and around 1.7 to 1.8. Accordingly, the organic EL element has problem in that confinement of light due to reflection at the interface between the layers occurs, and light extraction efficiency is not improved. Particularly, ITO (Indium Tin Oxide, Indium oxide doped with tin) is often employed as a transparent electrode, however, ITO has a high refractive index of around 1.9 and causes the problem as described above.
As a method for solving such a problem, a method is disclosed in Patent Document 1 described later in which two or more kinds of materials are heterogeneously dispersed in an electrode in the vicinity of an interface between the electrode layer and an organic layer or in an organic layer in the vicinity of an interface between an electrode layer and the organic layer. However, an electrode or an organic layer in which two or more kinds of materials are heterogeneously dispersed has problem that results in lowering electrical conductivity or transporting or blocking performance of electrons or holes, and therefore, it is especially difficult to employ such a technique in a transparent electrode. In the Patent Document 1, examples in which such a technique is applied to an organic layer or an electrode are not disclosed.
A technique is disclosed in Patent Document 2 described later in which a light loss preventing layer is provided between two layers having a large refractive index difference. However, provision of additional layers results in cost increase or load to productivity. Further, like the disclosure of the Patent Document 1, the provision of a light loss preventing layer between an electrode and an organic layer has problem that results in lowering electrical conductivity or transporting or blocking performance of electrons or holes. When the light loss preventing layer is provided on the surface of the electrode opposite the organic layer, only restricted effect is obtained, since there are no effects in an interface phenomenon effect between the organic layer and the transparent electrode. Thus, a technique is sought which is applied directly to a transparent electrode and improves light extraction efficiency without lowering electrical conductivity and the like.
Incidentally, a method in which metal nanowires are employed as a material for a transparent electrode, is disclosed in Patent Document 3 described later, however, there is neither disclosure nor suggestion in this document regarding light extraction efficiency in the organic EL element employing metal nanowires.